The primary goal of Project 2 is to determine how innate immune responses impact on the entry, infection, and replication of encephalitic flaviviruses within the central nervous system (CNS). Members of the Flavivirus genus include neurotropic viruses (e.g., West Nile (WNV), Japanese encephalitis (JEV), and tickborne encephalitis viruses) that continue to spread and cause human disease in new areas of the world. Although much work has focused on understanding flavivirus replication in cells, and on defining the virulence features of strains in vivo, less is known as to how the host innate immune mediators limit entry into the CNS and controls infection of neuronal target cells. Outside the CNS, innate immune cells recognize viral pathogen associated molecular patterns (PAMPs) and respond to infection with the orchestrated release of proinflammatory cytokines, which trigger cell-intrinsic antiviral responses and the induction of adaptive immunity. Because the CNS lacks secondary lymphoid tissues and cannot initiate adaptive immune responses, it relies, in part on innate responses of resident neural cells to limit viral invasion and the extent of infection. In preliminary studies, we have determined that types I (IF Nab) and 111 (lL-28) interferons (IFNs), modulate the ability of WNV to enter and infect the CNS. Thus, systemic and local IFN responses, which occur during WNV infection, induce brain microvascular endothelial cells (BMECs) to regulate blood-brain barrier (BBB) permeability and restrict viral entry. Once encephalitic flaviviruses enter the brain, several factors likely contribute to the regional heterogeneity and cellular distribution of infection that is observed in mouse models and human autopsy specimens. We hypothesize that innate immune responses in the CNS after flavivirus infection exhibit cell- and region-specific effects that restrict viral entry, infection, and injury of neurons. We also hypothesize that non-neuronal cells in the CNS (e.g., astrocytes, oligodendrocytes, and microglia) provide critical innate cues and produce inflammatory mediators that instruct neurons in developing specific innate immune programs to control virus infection. Insight into the cell-intrinsic and cell extrinsic processes by which the host controls flavivirus infection and minimizes neuronal injury is essential for developing strategies to contain virus spread, persistence, and disease.